In the steel-making and electric arc melting and/or refining of metals, it has been desired to reduce electric energy cost and wear of the end of outer periphery of graphite electrodes due to oxidation, thereby reducing the cost for electrodes. For suppressing the wear due to oxidization, it has been proposed and practiced to cool graphite electrodes. For the cooling of graphite electrodes in the refining of metal, for instance, there have been proposed a method and a device, in which of graphite electrodes which are connected successively upper ones are constructed such that their inside is cooled by cooling water, that is, they are constructed as water-cooled non-consumable electrodes, and only the remaining lower graphite electrodes which are connected via nipples to the lower end of and cooled from the non-consumable electrodes are consumed during melting and/or refining operations. For example, U.S. Pat. Nos. 4,416,014, 4,417,344 and 4,451,926 disclose structures, in which water-cooled non-consumable electrodes consist of hollow aluminum cylinders, and cooling water is introduced into these non-consumable electrodes to cool the wall surface thereof and graphite electrodes to cool the wall surface thereof and graphite electrodes connected to the lower end of these non-consumable electrodes.
Further, Japanese Patent Disclosures 501879/1985 and 501880/1985 disclose structures, in which water-cooled non-consumable electrodes consist of graphite pipes, and cooling water is introduced into the bore of these non-consumable electrodes.
Where the upper non-consumable electrodes are cooled to cool the lower graphite electrodes are connected thereto, wear of the end and outer periphery of the graphite electrodes due to oxidization can be suppressed to attain reduction of the cost for the electrodes.
However, when the graphite electrodes connected to the lower end of the non-consumable electrodes are worn out so that they are to be removed, the electrodes set has first to be removed from the electric arc furnace and transferred to an off-line before removing then from nipples and also removing, if necessary, the nipples from the non-consumable electrodes. When connecting new graphite electrodes, the nipples are first connected to the non-consumable electrodes, and then the new consumable electrodes are connected to the nipples. In this way, in the system where the lower consumable graphite electrodes are cooled from the upper water-cooled non-consumable electrodes, the replacement of worn-out lower consumable graphite electrodes requires works of transferring the electrodes set to the off-line and hard off-line labors of removing and connecting electrodes and nipples. These works and labors are very cumbersome. Further, if the removal and re-connection of consumable graphite electrodes are done repeatedly, it will lead to deformation or battering of and damage to the consumable and non-consumable electrodes and nipples, defectiveness of connection of electrodes and increase of the electric resistivity. In such cases, normal operation of melting and/or refining of metal will be impeded.
To solve the above problems, there has been proposed a cooling system, which does not use any water-cooled non-consumable electrode for cooling lower consumable electrodes graphite electrodes connected thereto. More specifically, Japanese Utility Model Publication 23,357/1984 discloses a cooling device, in which cooling water is blown against the surface of a graphite electrode extending upwardly from the cover of an electric arc furnace. This cooling device is as shown in FIG. 1. In the Figure, reference numeral 1 designates the cover of the electric arc furnace. A graphite electrode 2 vertically movably penetrates the cover 1, and a lower graphite electrode is connected to the lower end of this graphite electrode 2. The lower graphite electrode extends in the electric arc furnace to effect metal refinement, e.g., steel-making. Above the cover 1, an upper end portion of the graphite electrodes 3 is held by an electrode holder 3. The electrode holder 3 is provided at the bottom with a ring-like cooling ductline 4. The ductline 4 has a plurality of downwardly extending vertical pipes 5, which are in turn provided with nozzles 6 directed toward the graphite electrode surface. Cooling water supplied to the ring-like ductline 4 descends along the vertical pipes 5 to be blown out from the nozzles 6 against the outer periphery of the graphite electrode for the cooling thereof.
In the cooling device shown in FIG. 1, however, cooling water is jet from each nozzle 6 in the horizontal direction. Therefore, when it strikes the outer periphery of the graphite electrode 2, a considerable quantity of it is spattered. Because of the great quantity of spattered cooling water, the electrode holder 3 and cover 1 are subject to serious contamination and damage, so that the cooling device is practically infeasible. Further, since only a slight proportion of the jet cooling water contributes to the cooling, it is necessary to use an extraordinarily great quantity of cooling water, which is undesired very much in view of the economy. Still further, a plurality of vertical pipes 5 extends downwardly to a very large extent from the ring-like cooling ductline 4. These long vertical pipes 5 constitute an obstacle when removing the cooling device for replacement of electrodes, that is, they dictate very cumbersome works for the electrode replacement.
The cooling device shown in FIG. 1 has a yet further drawback. Since the ring-like cooling ductline 4 is provided such that it surrounds the outer periphery of the graphite electrode 2, it shields electromagnetic forces to cut off a considerable portion of current passed through the graphite electrode 2. This presents serious problems in the operation of the electric arc furnace. Usually, for its operation an electric arc furnace uses three graphite electrodes in correspondence to a three-phase AC power source. For cooling these graphite electrodes, the cooling device as shown in FIG. 1 is provided for each of them. Since each cooling ductline 4 is ring-like, the individual graphite electrodes 2 are mutually electromagnetically influenced by one another. Meanwhile, since each cooling ductline 4 shields electromagnetic forces, current through each graphite electrode 2 is cut off. Therefore, the electrode consumption is greatly increased to obtain sufficient heating of metal.